It is well known to spray apply hydraulic cementitious slurries onto metal structural members in order to provide a heat resistant coating thereon. U.S. Pat. Nos. 3,719,513 and 3,839,059 disclose gypsum-based formulations which contain, in addition to the gypsum binder, a lightweight inorganic aggregate such as vermiculite, a fibrous substance such as cellulose and an air entraining agent for such purpose. U.S. Pat. No. 4,751,024 teaches sprayable cementitious compositions containing shredded polystyrene as a lightweight aggregate in fireproofing compositions. Such slurries are generally prepared at ground level and are pumped to the point of application, where they are spray applied to the substrate. Often the point of application exceeds 20 or 30 stories where high rise construction is involved, and the slurry is generally applied through a spray nozzle.
Slurries must possess a number of important properties to be suitable as heat resistant coatings. First, they must be sufficiently fluid to be pumped easily and to great heights. Second, they must retain a consistency sufficient to prevent segregation or settling of ingredients and provide an adequate "yield" or volume of applied fireproofing per weight of dry mix. Third, they must adhere to the metal the structure member is comprised of, both in the slurried stated and after setting. Fourth, the slurry must set without undue expansion or shrinkage which could result in the formation of cracks that can deter from the insulative value of the coating.
U.S. Pat. No. 4,934,596 of Hilton et al., owned by the common assignee herein, the disclosure of which is incorporated herein by reference, teaches a slurry distributor for distributing a low viscosity fluid such as an accelerator into a viscous, hydraulic slurry. Specifically, the low viscosity fluid is introduced into the flowing high viscosity slurry, and means is provided in the distributor for directing the low viscosity fluid so that it may be substantially evenly dispersed (such as with air) with the slurry onto the steel member. In one embodiment, the means for directing the low viscosity fluid is an air stem appropriately positioned in the distributor to intersect the flow of the slurry.
One difficulty encountered in spray applying multi-component blends such as cementitious slurries and set accelerators therefor is variable flow rates, in view of the back pressures which can develop in the system. If such back pressures are not carefully regulated, varying amounts of accelerator, for example, may be added to the slurry, causing inconsistencies in the resulting fireproofing coating.